Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: poles on ring magnets

Learn more physics!

Most recent answer: 05/06/2014
For a ring shaped magnet, it's not possible to have a pole on the outer side of the ring and have the other pole on the inner side of the ring, right? I think I may have an approximate idea why, but I'm not sure so I'm hoping for a proper explanation.
- Shang-Yan Yang (age 19)
Boston, MA, America

Actually, it is possible. For example, take a big collection of bar magnets and arrange them in a circle, with all the S poles pointing in. It's not a magnet type that thas many uses, but it's certainly possible.

Mike W.

(published on 05/04/2014)

Follow-Up #1: frictionless magnetic bearings?

Oh, I didn't think of that. So why can't this be used to create some kind of friction-less rotational bearing? Or if they do exist, are they just too expensive to be placed in everyday objects?
- Shang-Yan (age 19)
Boston, MA, America

I think you're thinking of some way to support some magnetic object in the middle of the ring, so that it would float and spin without any contact friction with the support. Earnshaw's Theorem says there's no way with gravity, electrostatic forces, and ordinary magnets to make a stable static structure. You can make a stable structure if you include diamagnetism- the tendency of some materials to repel magnetic fields. Diamagnetism is very weak, except for typical superconductors, which are strongly diamagnetic. So you could make a superconducting bearing in the ring magnet, with very low friction. We don't have any materials that superconduct at room temperature, however, so things would have to be kept cold. There would be some friction due to contact with the cooling gases, but it would be small. 

(published on 05/06/2014)

Follow-up on this answer.